How could I mock this and make sure to not get back null?

I have the following class that I want to unit test:

@RequiredArgsConstructor
@Service
public class Service {
private final WebClient.Builder builder;
private WebClient webClient;
@PostConstruct
public void init() {
searchUri = "/search-uri";
webClient = builder.baseUrl(searchUri)
.defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
.build();
}    
public ResponseSpec search() {
return webClient
.get()
.uri(uriBuilder ->
uriBuilder
.path("/search-uri")
// alot of query param, not important
.build()
)
.accept(MediaType.APPLICATION_JSON)
.acceptCharset(StandardCharsets.UTF_8)
.retrieve();
}
}

This is my test class:

@ExtendWith(MockitoExtension.class)
@RunWith(JUnitPlatform.class)
public class ServiceTest {
@InjectMocks
private Service service;
@Mock
private WebClient webClient;
@Mock
private Builder builder;
@Test
public void testSearch() {
when(builder.baseUrl(Mockito.anyString())).thenReturn(builder);
when(builder.defaultHeader(Mockito.anyString(), Mockito.anyString())).thenReturn(builder);
when(builder.build()).thenReturn(webClient); 
issuerServiceImpl.init();
WebClient.RequestHeadersUriSpec uriSpecMock = mock(WebClient.RequestHeadersUriSpec.class);
WebClient.RequestHeadersSpec headersSpecMock = mock(WebClient.RequestHeadersSpec.class);
WebClient.ResponseSpec responseSpecMock = mock(WebClient.ResponseSpec.class);
when(webClient.get()).thenReturn(uriSpecMock);
lenient().when(uriSpecMock.uri(Mockito.any(URI.class))).thenReturn(headersSpecMock);	   
issuerServiceImpl.searchIssuers("");
}
}

An exception (NullPointerException) happens in the line of .accept(MediaType.APPLICATION_JSON) because the uri() returns null and the code tries to call method .accept() on a null object.

I am not let to change the Service class. I can change only my test class. Not sure how could I make it to work.

Edit: I made it to work, read answers.

So actually I changed this:

lenient().when(uriSpecMock.uri(Mockito.any(URI.class))).thenReturn(headersSpecMock);

to this:

lenient().when(uriSpecMock.uri(Mockito.any(Function.class))).thenReturn(headersSpecMock);

And it works good.

I think this is the problem

when(uriSpecMock.uri(Mockito.any(URI.class)))

you mocked uri to return something when it receives something that's URI class but you're using a lambda function.
The method signature is

S uri(Function<UriBuilder, URI> uriFunction);

Try changing it to

when(uriSpecMock.uri(Mockito.any(Function.class)))

Also check this for future use, quite a good article

https://www.baeldung.com/spring-mocking-webclient